Battery management systems are used to safely optimise the efficient use and charging of batteries comprising one or more cells.
Typically, battery management systems comprise components which:                measure the state of charge i.e. the amount of stored energy remaining in the battery;        measure the state of health measurement i.e. the life expectancy of the battery;        ensure safe battery operation;        control the charging of the battery by regulating the charging current and voltage; and        provide cell balancing to ensure that the maximum energy is stored and delivered without activating protection circuitry.        
Presently many different electronic circuits are employed to implement the functions described above. In general, cost constraints mean that not every battery management system employs all the above functions. Semiconductor manufacturers have developed specific electronic integrated circuits that provide one or more of them in an attempt to reduce cost and minimise solution size. Examples of devices of this type are Fuel Gauging IC's that provide state of charge (SoC), Protection IC's that monitor the safe operation of the battery, Passive Cell Balancer IC's that ensure safe charging of multiple series connected battery cells, and Charger IC's that control the battery's charger unit. It therefore takes a number of integrated circuits and additional discrete circuitry to build a complete battery management system.
A key feature of any battery management system is the ability to provide an accurate indication of its state of charge either as a percentage of absolute or relative capacity or expressed in Amp Hours (Ahr) or as a time to empty figure. State of the art secondary cell chemistries are commonly based on Lithium as it has the highest commercially available energy density. Lithium secondary cells have a near 100% coulometric efficiency which offers the potential for the accurate determination of state of charge through the integration of cell current. However, there are a number of mechanisms and environmental factors that can introduce measurement error into the coulometric SoC determination process.
In use, the battery pack SoC system will drift due to measurement error accumulated over time, for example, due to effects such as self leakage and temperature sensitivity of measurement system.
Typically, at time of manufacture, each battery is given its specified State of Charge status, which is 100% by default. This information is permanently recorded as an ‘absolute’ value into the pack and does not change. With each charge, the battery resets to the full-charge status. During discharge, the energy units (coulombs) are counted and compared against the 100% setting ‘absolute’ value. A perfect battery would indicate 100% on a calibrated fuel gauge. As the battery ages and the charge acceptance drops, the SoC decreases. The discrepancy between the factory-set 100% and the delivered coulombs on a fully discharged battery indicates the SoH.
A fuel gauge combines SoC and SoH information to allow a user to determine the amount of useable charge that is available in the battery.
There are a number of problems associated with providing accurate fuel gauges. Typically cell capacity will reduce with time and number of charge discharge cycles therefore cell age must be compensated for when determining the State of Charge.
The effective cell capacity is related to cell temperature with a strong fall off at cell temperatures below 0° C. and temperature must be compensated for when determining the State of Charge.
There are a number of low level current drains on each cell that the coulometric system is unable to measure. Such currents include the cell internal self leakage current and the quiescent current of the battery management system. Also, the electronic measurement circuits drift with time and temperature. These must also be compensated for when measuring State of Charge.
As mentioned above, typical fuel gauges work from an initial State of Charge calibration done at the time of manufacture. This process is time consuming and costly. In general, battery pack manufacturing costs would decrease if this step could be omitted from the manufacturing process.
While prior art systems attempt to deal with cell ageing and temperature problems, little attention has been paid to the mitigation of fuel gauging measurement system drift and the reduction in battery pack manufacturing cost and time.
Present SoC systems compensate for cell ageing through impedance tracking techniques that link cell age to its impedance. Temperature is also compensated through algorithms based on measured battery pack temperature.